The oil sands of the Fort McMurray region of Alberta are presently being exploited by two large commercial operations. The process practiced in these operations involves four broad steps, namely:
mining the oil sand;
extracting the bitumen from the mined oil sand using a process known as the `hot water process`, to produce bitumen in the form of a froth contaminated with water and solids;
purifying the froth to separate the water and solids from the bitumen; and
upgrading the purified bitumen in a coking facility to produce products which are suitable for a conventional refinery.
The present invention has to do with the purifying step. However, in order to understand the problems solved by the invention, it is first necessary to review the steps of the hot water process and the conventional froth purification process.
As a beginning point, it needs to be understood that oil sands comprises relatively large quartz sand grains, each grain being encapsulated in a thin sheath of connate water. The water contains minute clay particles (referred to as `fines`). The bitumen is positioned in the interstices between the water-sheathed grains of sand.
In the first step of the hot water process, the mined oil sand is mixed in a rotating horizontal cylindrical drum (or `tumbler`) with hot water (80.degree. C.) and a small amount of NaOH (referred to as `process aid`). Steam is sparged into the slurry at intervals along the length of the drum, to ensure that the exit temperature of the resultant slurry is about 80.degree. C.
The drum is slightly inclined along its length, so that the mixture moves steadily therethrough. The retention time is about 4 minutes.
This tumbling step is referred to as `conditioning`. It involves heating of the bitumen and displacement, by water addition, of the bitumen away from the sand grains. Many of the released bitumen globules become aerated by forming films around air bubbles entrained in the tumbler slurry. Conditioning also involves reaction between the NaOH and bitumen to produce surfactants which facilitate the bitumen-release and subsequent flotation/settling steps.
On leaving the tumbler, the conditioned slurry is screened, to remove oversize rocks and lumps, and diluted with additional hot water. The resulting water/bitumen ratio is about 6:1.
The diluted slurry is then introduced into a large thickener-like vessel having a cylindrical upper portion and a conical lower portion. The vessel is referred to as the `primary separation vessel` or `PSV`. Here the diluted slurry is retained for about 45 minutes under quiescent conditions. Under the influence of gravity, the sand grains sink, are concentrated in the conical portion and are discharged as `primary tailings` through a valve and line connected to the lower apex of the vessel. The bitumen globules, rendered buoyant by air attachment, rise to the surface of the PSV and form a froth. This froth is called `primary froth` and typically comprises:
66.4% by wt. bitumen PA1 24.7% by wt. water PA1 8.9% by wt. solids PA1 23.8% by wt. bitumen PA1 58.7% by wt. water PA1 17.5% by wt. solids. PA1 41.4% by wt. bitumen PA1 46.2% by wt. water PA1 12.4% by wt. solids. PA1 57.3% by wt. bitumen PA1 34.2% by wt. water PA1 8.4% by wt. solids. PA1 99.0% by wt. bitumen PA1 % by wt. water PA1 1.0% by wt. solids. PA1 59.4% by wt. bitumen PA1 37.5% by wt. naphtha PA1 4.5% by wt. water PA1 0.4% by wt. solids
The primary froth is skimmed off and recovered in a launder. In between the layer of sand tailings in the base of the vessel and the layer of froth at the top, there exists a watery slurry referred to as `middlings`. The middlings contain fines and globules of bitumen which are insufficiently buoyant to reach the froth layer.
A stream of middlings is continuously withdrawn from the PSV. These middlings are treated in a series of sub-aerated flotation cells. In these cells, the middlings are vigorously aerated and agitated, with the result that contained bitumen is forced to float and form a dirty froth referred to as `secondary froth`. This secondary froth typically comprises:
To reduce the concentration of water and solids in the secondary froth, it may be retained in a settling tank to allow some of the contaminants to settle out. The `cleaned` secondary froth typically comprises:
The primary and secondary froths are then combined to provide the product of the hot water extraction process. The `combined froth` typically comprises:
This stream is too contaminated to be used as feed to the downstream upgrading circuit. This latter circuit requires a feed typically comprising:
So the combined froth product requires purification (or water and solids removal) before it can be fed to the upgrading circuit. Heretofore, this purification has been obtained by using what is referred to as `two stage dilution centrifuging`. This operation involves:
1. Diluting the combined froth with naphtha. This is done to reduce hydrocarbon phase viscosity and increase the density difference between the hydrocarbon phase (bitumen dissolved in naphtha) and the water and solids phase (referred to jointly as `sludge`);
2. Passing the diluted froth through a low-speed scroll centrifuge, to remove the coarse solids and some of the water as a cake, which is discarded; and
3. Passing the scroll centrifuge product through a high-speed disc centrifuge to remove fine solids and most of the balance of the water. The disc centrifuge product typically analyzes at:
The naphtha diluent and any contained water is then distilled out of the disc centrifuge product to produce the purified bitumen product for advancing to the upgrading process.
The described dilution centrifuging process has been used because it is capable of producing a bitumen product of the desired quality. But it is an operation that is exceedingly expensive to maintain and operate due to the erosive nature of the feed and the rotating character of the centrifuges. For example, in use, the flights of the scroll centrifuges wear badly, even though they are formed of ceramic, and the brittle ceramic flights commonly break and put the machine out of balance. In the case of the disc centrifuges, their sludge discharge nozzles are subject to rapid wear and the separation interface between product and reject in the stack of discs can easily be `lost`, with the result that a significant amount of bitumen is lost with the tailings. In addition, a large number of the machines must be used, with attendant consumption of very large amounts of electrical energy.
Thus, there has long been a need for a viable alternative to the dilution centrifuging circuit for purifying bitumen froth.
The present invention involves a circuit of interconnected known devices, namely mixers and inclined plate settlers (`IPS`).
An inclined plate settler comprises a stack of parallel, spaced apart, solid plates, inclined downwardly from the horizontal and mounted within a containing vessel. Each space between a pair of plates forms a discrete settling zone. The feed mixture to be separated is distributed into the spaces, at a point between their longitudinal ends. The light components of the mixture rise to the underside surface of the upper plate. These light components then travel up said underside surface and are collected and recovered at the upper ends of the plates. The heavy components of the mixture sink towards the uppermost surface of the lower plate and follow it downwardly, to be collected and recovered at the lower ends of the plates.
A mixer can take any of various forms--the present work involved simply a cylindrical container having a submerged driven impeller positioned therein.